1. Field of the Invention
The present invention relates to a projection cathode-ray tube in which an image on a phosphor layer is enlarged and projected on a screen located at a given distance ahead through a projection lens in front of said phosphor layer.
2. Description of the Prior Art
In a television set with a color cathode-ray tube of a shadow mask type widely utilized at present, its screen size is considered to be limited to approximately 30" to 40" at maximum principally because of the structural restrictions. As a result, as one means for receiving a video image and the like with a larger screen size, a projection type television set 1 as shown in FIG. 1 has been developed and is widely utilized nowadays.
In such a projection type television set 1, monochromatic images in blue, green and red respectively obtained by small-sized monochromatic cathode-ray tubes 2, 3 and 4 of approximately 5" to 8" size are enlarged and projected on a screen 6 located at a given distance ahead by means of projection lens units 5, so that a color image of a large size can be obtained on the screen 6. Since the size of the screen 6 is generally 40" to 70", the images on the small-sized monochromatic cathode-ray tubes 2, 3 and 4 are projected to be 50 to 100 times larger on the screen 6. Therefore, in such a projection type television set 1, it is an important point in performance how to obtain a sufficiently bright image on the screen 6. For this reason, constant efforts have been made for improvement of phosphor materials for use in projection cathode-ray tubes, application of a structure of a cathode-ray tube enabling highly loaded operation, improvement of the screen 6 and the projection lens unit 5, and the like.
One of the major factors hindering improvement of the brightness of the projected image in the projection type television set 1 is a low efficiency for gathering luminous flux into the projection lens unit 5 from the monochromatic cathode-ray tubes 2, 3 and 4. This problem will be described in more detail with reference to FIG. 2.
FIG. 2 is a sectional structural view showing the monochromatic cathode-ray tube 2, 3 or 4 of the projection type television set 1 and the projection lens unit 5 in front of the tube. The monochromatic cathode-ray tube 2, 3 or 4 comprises a vacuum vessel 10 and an electron gun 13 enclosed in the vessel 10. On the inner surface of the face plate 7 constituting a portion of the vacuum vessel 10, a phosphor layer 8 is formed and on the phosphor layer 8, a metal-back film 9 made of evaporated aluminum serving as a high-voltage electrode and a reflective film is formed. By the energy of an electron beam from the electron gun located behind the metal-back film 9, the phosphor layer 8 is excited so that output of phosphorescent light can be obtained.
The projection lens units 5 are disposed close to the above stated face plates 7 of the monochromatic cathode-ray tubes 2, 3 and 4, respectively. The projection lens unit 5 is structured as a compound lens having 3 to 8 optical lenses generally incorporated in a barrel 12. The projection lens unit 5 shown in the drawing is an example of a compound lens comprising six lenses. In the case of the projection lens unit 5 as described above, it is difficult to select a large lens diameter as compared with the face plate 7 of the monochromatic cathode-ray tube 2, 3 or 4, because of the limited conditions as to the aberration, the cost and the space. As a result, the usable angle with which light emitted from the phosphor layer 8 can be accepted into the projection lens unit 5 is limited to an extremely small range.
For example, as for the light emission at the center of the phosphor layer 8, the range of the optically usable outermost light paths is shown as lc. The angle .theta..sub.1 formed by the usable outermost light path with respect to a normal perpendicular to the phosphor layer 8 at the emission point is in the range of .theta.=15.degree. to 20.degree. approximately, which differs a little depending on the structure of the projection lens unit 5.
As for the light emission in a peripheral portion of the phosphor layer 8, the range of the optically usable outermost light paths is shown as le. The angles .theta..sub.2 and .theta..sub.3 formed by the usable outermost light paths le with respect to a normal perpendicular to the phosphor layer 8 are approximately 15.degree..ltoreq..theta..sub.2 .ltoreq.20.degree. and 25.ltoreq..theta..sub.3 .ltoreq.30.degree., respectively.
Accordingly, both in the central portion and in the peripheral portion of the phosphor layer 8, any luminous flux emitted at a divergent angle larger than 30.degree. with respect to a normal perpendicular to the phosphor layer 8 is useless flux which cannot be transmitted through an usable light path of the projection lens unit 5.
FIG. 3 shows orientation dependence of the luminous flux from the phosphor layer 8 excited by an electron beam EB in a conventional monochromatic cathode-ray tube. In this case, the phosphor layer 8 serves as a nearly perfect diffuser and accordingly, the Lambert law applies. The curve K in FIG. 5 shows the relative luminous intensity with respect to the divergent angle in such case. In the following, we will describe the efficiency for accepting the emitted light into the projection lens unit 5 in case of the phosphor layer 8 serving as a nearly perfect diffuser as described above.
Referring to FIG. 3, assuming that a minor emission area at a point P in the phosphor layer 8 is .DELTA.S, that the brightness of the area in a direction inclined by .theta. with respect to the normal is L.sub..theta., and that the luminous intensity in the direction .theta. at a sufficiently long distance as compared with .DELTA.S is I.sub..theta., the following equation is obtained. EQU I.sub..theta. =.intg.L.sub..theta.. cos.theta.ds=L.sub..theta.. cos.theta...DELTA.S (I)
If the emission area is a perfect diffuser, L.sub..theta. is constant independently of the angle .theta. and can be represented as follows: EQU L.sub..theta.=L=constant (II)
Now, assuming that the luminous flux emitted forward from the perfect diffuser .DELTA.S at the point P into a cone with an apex angle of 2.theta. is .phi..sub..theta., the following equation is established. ##EQU1##
By substituting the equations (I) and (II) into the equation (III), the following equation is established. ##EQU2## Accordingly, by substituting ##EQU3## into the equation (IV), the total luminous flux .phi..sub.T emitted forward from .DELTA..sub.S is obtained as follows: EQU .phi..sub.T =.pi.L.DELTA.S (V)
Consequently, if the luminous flux emitted into the cone having the apex angle 2.theta., out of the total luminous flux emitted from .DELTA.S at the point P shown in FIG. 3 is accepted into the projection lens unit 5, the efficiency for accepting luminous flux, namely the light gathering efficiency .eta. is represented by the following equation, based on the equations (IV) and (V). ##EQU4##
FIG. 4 shows a relation between the angle .theta., namely, the angle for accepting light from a monochromatic cathode-ray tube into the projection lens unit 5 and the light gathering efficiency. If the accepting angle is .theta.=30.degree. as in the above described conventional projection type television set, the light gathering efficiency is 25%, the remaining luminous flux of 75% never contributing to the brightness of the projected image on the screen.